In optical devices, for example light microscopes, in which a specimen for examination is illuminated by a light source, it is generally desirable if the brightness of the light source is modifiable so that the illumination conditions can be optimally adapted to the type of specimen being examined and the observation method being used (e.g. bright-field, polarization, interference contrast, or phase contrast). Incandescent lamps or halogen lamps are preferably used as the light sources in this context, since they are available from numerous manufacturers in a wide variety of configurations in terms of output, operating voltage, filament shape, service life, and color temperature.
In microscopes that operate with an incandescent lamp as the light source, the brightness of the light source is usually modified by the fact that the electrical power delivered to the light source is increased or lowered by way of a corresponding reduction in the electrical voltage or electrical current delivered to the incandescent lamp.
Because the behavior of incandescent lamps is similar to that of a black body, there exists in the context of this type of brightness modification the problem that upon a reduction in the delivered lamp power, the color temperature of the spectrum emitted by the incandescent lamp shifts out of the blue spectral region toward the red spectral region; this is generally also referred to as a “red shift” and as a rule is considered disadvantageous in microscopes. For example, a reduction from 100% to 80% of the rated voltage in the voltage applied to an incandescent lamp results not only in a reduction in light flux of approximately 50% but also, in similar fashion, in a lowering of the color temperature T from 3200 K to 2950 K.
DE 41 42 925 A1 discloses a method and an apparatus for color-neutral brightness regulation of an illumination system for a microscope in which the brightness is set to a desired value by way of the electrical current delivered to a main light source, and the change in color temperature associated therewith is simultaneously compensated for by means of a corresponding regulation of the lamp current of a secondary light source (in a second lamp housing) having a suitable downstream optical filter which has a fixed transmittance. Although heating of the stand is decreased, in the apparatus described, by the arrangement of the secondary light source outside the microscope stand, in this apparatus all the dissipated power of an incandescent bulb occurs as heat.
DE 195 13 350 A1 describes a filter that comprises an opaque filter disk having openings of different sizes in various regions of the filter disk. Because of the different sizes of the openings, the filter has a different transmission in the various regions of the filter disk, so that by rotation of the filter disk introduced into the illumination beam path of an optical instrument, the brightness of an incandescent lamp can be modified in color-neutral fashion. For color-neutral brightness regulation, however, the incandescent lamp must also always be operated in the vicinity of the rated voltage, once again resulting in undesired heating of the stand and the filter disk and in a shorter service life for the incandescent lamp (as compared to operation at less than the rated output).
In addition, DE 198 32 665 A1 describes a homogenization filter for an optical radiation field of an illumination beam path that, in coaction with a diffusion disk, results in a homogenization of the light intensity in a downstream image plane. The homogenization filter comprises a transparent substrate onto which a grid of opaque planar elements is applied by evaporative deposition, by means of a perforated stencil arranged in front of the substrate or a perforated layer applied onto the substrate in a photolithographic procedure.